Various liquid dispensing modules have been developed for the precise application of a heated liquid, such as a thermoplastic hot melt adhesive, on a substrate. In many dispensing applications, the flow of heated liquid must be periodically interrupted to sharply delimit the leading and trailing edges of individual application zones in a pattern of heated liquid applied on the substrate. To that end, most liquid dispensing modules have an open position in which heated liquid is discharged and a closed position in which the flow of heated liquid is blocked. Rapid cycling between the open and closed positions interrupts the flow and provides the high-speed intermittent flow discontinuities required to generate the pattern of heated liquid.
For modulating the flow of heated liquid, liquid dispensing modules generally include an actuator and a dispenser body having a valve seat and a valve plug operatively connected with the actuator for movement relative to the valve seat. In the open position, the actuator operates to space the valve plug from the valve seat so that heated liquid can flow through a series of internal passageways to a discharge orifice in the dispenser body. In the closed position, the valve plug engages the valve seat so that flow is blocked. Liquid dispensing modules are characterized by an intrinsic cycle time, which includes the time required to actuate from the closed position to the open position and the time required to return to the closed position. The liquid dispensing module is maintained in the open position for a dispensing time sufficient to tailor the application zones of the desired application pattern.
Liquid dispensing modules are often pneumatically actuated with pressurized fluid to provide the open and closed positions. In such modules, the actuator includes a solenoid valve that regulates the application of the pressurized fluid to an air cavity, an air piston displaced in response to the application of pressurized air to the air cavity, and an air piston housing in which the air piston and air cavity are disposed. The air piston is operatively coupled with the valve plug in the dispenser body and provides at least the motive force that produces the open position of the module. The shortest cycle times are achieved when the solenoid valve is attached in direct contact with the air piston housing.
The dispenser body of the liquid dispenser module is often fluidically coupled with a liquid distribution manifold. Heated liquid from a heated liquid supply flows through various internal passageways in the liquid distribution manifold and the liquid dispensing module before being applied on the substrate. Heated liquid flowing through the liquid distribution manifold and the liquid dispensing module will attempt to thermally equilibrate with the surrounding walls of the passageways. If the heated liquid cools below a threshold temperature, it may not remain flowable and/or molten or may not have the desired properties when applied on the substrate. To avoid the detrimental effects of cooling, the liquid distribution manifold is provided with heating elements that elevate the temperature of the manifold. Heat transfer from the liquid distribution manifold heats the liquid dispensing module. Alternatively, the liquid dispensing module may incorporate independent heating elements. For specific dispensing operations in which the heated liquid is a hot melt adhesive, it is desirable maintain the liquid distribution manifold and the liquid dispensing module at an operating temperature exceeding about 250° F. and as high as about 400° F.
Significant heat transfer also occurs from the liquid distribution manifold and the dispenser body to the air piston housing. Because the solenoid valve is in thermal contact with the air piston housing, this transferred heat can be further transferred from the air position housing to the solenoid valve. The transferred heat elevates the operating temperature of the solenoid valve, which can approach the operating temperature of the liquid distribution manifold. If the operating temperature rises above a certain threshold temperature, the solenoid valve cannot operate properly and may malfunction, suffer permanent damage, or fail.
The designs of certain conventional liquid dispensing modules attempt to reduce the heating of the solenoid valve by spacing it physically from the air piston housing. To do so, a nipple or a length of tubing must provided to fluidically couple an air outlet of the solenoid valve with an air inlet of the air piston housing leading to the air cavity. The nipple or tubing reduces the path for conduction of heat from the actuator to the housing of the air cavity. However, the volume of the air space within the nipple or tubing increases the effective air volume of the air cavity that must be pressurized in order to actuate the air piston. The increase in the effective air volume increases the cycle time of the actuator. In such applications, the smallest effective air volume for conventional air cavities is greater than 2170 mm3. The fastest of conventional liquid dispensing modules designed with such effective air volumes have cycle times, excluding the time required for switching the flow of pressurized fluid within the solenoid valve and the actual dispensing time, that exceed 9 milliseconds. It follows that simply spacing the solenoid valve from the housing containing the air cavity with a nipple or a length of tubing is not an adequate solution for reducing the heating of the solenoid valve in those dispensing applications requiring a cycle time of 9 milliseconds or less.
The transfer of heat from the dispenser body and the distribution manifold also reduces the useful lifetime of the solenoid valve. Manufacturers of common solenoid valves recommend a maximum temperature for continuous operation of less than about 140° F. If the solenoid valve is equipped with custom high-temperature seals, the heat-tolerance of the valve increases so that it can operate continuously at temperatures greater than 140° F. and as high as about 225° F. However, the addition of high-temperature seals to the solenoid valve further increases the cycle time because of the softness of the material composing the high-temperature seals. Therefore, equipping a solenoid valve with high-temperature seals permits the valve to operate over a larger temperature range but presents a significant liability for high-speed dispensing operations. Moreover, even if a solenoid valve is equipped with such high-temperature seals, it still cannot operate reliably if heated above about 225° F.
What is needed, therefore, is a liquid dispensing module for dispensing a heated liquid that can reduce the transfer of heat from the liquid dispensing module and the heated liquid distribution manifold to the pneumatic actuator. Also needed is a liquid dispensing module having a reduced cycle time for dispensing liquids, including heated liquids.